Pressure gradient hydrophone



F. M ASSA Jan. 9, 1968 PRESSURE GRADIENT HYDROPHONE Filed Aug. 23, 1965 FIG. 8

E m T m B D 6 FREQUENCY United States Patent C) 3,363,228 PRESSURE GRADIENT HYDROPHONE Frank Massa, Cohasset, Mass., assignor to Massa Division, Dynamics Corporation of America, Hingham, Mass. Filed Aug. 23, 1965, Ser. No. 481,809 19 Claims. (Cl. 340-) This invention is concerned with an improved construction of a hydrophone of the pressure gradient type which can be used under water at relatively great depths.

One object of my invention is to provide a pressure gradient hydrophone that will operate satisfactorily throughout the lower portion of the audible frequency region.

Another object of my invention is to provide a stiffness controlled vibrating system over the frequency range of operation in order that the phase relationship between the sound pressure and the generated voltage will remain fixed from unit to unit.

A still further object of my invention is to provide a low cost mechanical structure which achieves the desired performance requirements.

Another object of my invention is to neutralize the hydrostatic pressure before it is transferred to the active transducer element, so that the element is effectively subjected only to the sound pressure oscillations when it is immersed in deep water.

These and other objects of the invention are set forth with particularity in the appended claims. However, for a better understanding of the invention itself, together with further features and advantages thereof, reference is made to the accompanying description and drawings in which is shown an illustrative embodiment of one form of my invention. This is understood by the following figures.

FIG. 1 is a longitudinal cross section of a hydrophone incorporating one embodiment of my invention.

FIG. 2 is a view taken along the section 22 of FIG. 1.

FIG. 3 is a view similar to FIG. 2 showing a different electrode arrangement for obtaining improved sensitivity for the hydrophone.

FIG. 4 is another view similar to FIG. 2 showing a still different electrode arrangement for obtaining still further improvement in hydrophone sensitivity.

FIG. 5 is a View taken along the section 55 of FIG. 4 showing the exaggerated deflection curve of the clamped circular transducer element when a force is applied at the center.

FIG. 6 is another View similar to FIG. 2 showing another modification of electrode arrangement for obtaning improved sensitivity for the hydrophone.

FIG. 7 is a view taken along section 77 of FIG. 6 and shows details of the electrode arrangement.

FIG. 8 shows the relationship between the sensitivity and frequency for the pressure gradient hydrophone.

FIG. 9 shows the directional response characteristic of the hydrophone in a plane containing the longitudinal axis of the unit.

Referring more particularly to FIG. 1 and FIG. 2, the reference character 1 represents a ring-like housing structure having a shape as illustrated by the cross section. A bi-laminar disc, comprising two bonded plates 2A and 2B of piezoelectric material such as polarized barium titanate or polarized lead zirconate titanate, is cemented at its periphery to the recessed shoulder in part 1, as illustrated. The bi-laminar assembly includes metallic electrodes 3 and 4 on the external surfaces of the ceramic and similar electrodes 33 and 44 at the inner bonded faces of the ceramic discs. The polarization of the two individual portions 2A and 2B of the bonded assembly is of opposite polarity the same as is illustrated by the and signs in the plate arrangement of FIG. 7. This polarization causes the combined series voltages across both ceramic plates to be additive when the center of the piezoelectric assembly is deflected. For the electrode arrangement shown in FIGS. 1 and 2, the diameter of the circular electrodes should not exceed approximately 55 of the diameter of the unsupported area of the composite bi-laminar structure. If a larger diameter of electrode is used, the outer portion will generate a voltage of opposite polarity to the center section and thereby reduce the sensitivity. An insulated conductor 5 passes through an insulating bushing 6 through the wall of part 1 and is electrically connected to electrode 3. Another conductor 7 similarly passes through insulating bushing 8 and is attached to electrode 4. When the bi-laminar disc is cemented at its periphery to the housing 1 as described, an oscillatory vibration imparted to the center of the piezoelectric disc assembly will generate corresponding alternating voltages across the conductors 5 and 7.

In FIG. 3 is shown an improved arrangement of electrode configuration to obtain increased sensitivity from the hydrophone. In this improved design, the electrodes are annular in shape as illustrated by 3A. For optimum sensitivity, the inside diameter of the annular electrode 3A should not be less than approximately 55% of the outside diameter of the electrode and the outside diameter should be approximately equal to the diameter of the unsupported area of the bi-laminar assembly. The improvement in sensitivity realized by the electrode arrangement of FIG. 3, as compared to FIG. 2, results from the increased stress which is created in the outer portion of the clamped disc assembly when it is deflected as compared to the stress developed near the center portion of the assembly. An additional advantage results from the use of the annular electrodes of FIG. 3 because a larger area of the ceramic is covered which in turn results in a substantial increase in the capacitance of the assembly.

FIGS. 4 and 5 illustrate an arrangement for using the entire area of the bi-laminar assembly in which both the electrodes of FIG. 2 and FIG. 3 are employed to achieve maximum efliciency of operation. In using this dual electrode arrangement for the electrical connections illustrated, the polarization of the individual plates 2A and 2B must be in opposite directions in the regions beneath the center electrodes 3 and 4 as compared to the regions beneath the outer annular electrodes 3A and 4A as illustrated by the and signs in FIG. 5. This reversal in polarization will develop voltages which are in phase across the electrodes 3 and 3A during deflection of the assembly and conductor 5 can be attached to both electrodes 3 and 3A in parallel as shown. The separation between the outside diameter of electrode 3 and the inside diameter of electrode 3A should occur at approximately 55% of the full unsupported diameter of the ceramic bilaminar assembly because this is the region of zero stress as illustrated by the exaggerated deflection curve of the assembly in FIG. 5. The arrangement of the electrodes 4 and 4A follow the same general configuration as was described for electrodes 3 and 3A which permits the conductor 7 to be attached as shown.

FIG. 6 and FIG. 7 illustrate an electrode arrangement that permits a simple series electrical connection between the inner and outer sections of the assembly as is indicated by connecting the conductor 5A to the electrode 3 in FIG. 7 and connecting the conductor 7A to the electrode 3A. For the arrangement illustrated in FIG. 7, the polarization of the plate 2A is in the same direction over its entire area and thus avoids the need for reversed polarization over the center and outer portions of the plate as is required by the electrode configuration in FIG. 5. The electrode 4C covers the entire exposed surface of the plate 2B in FIG. 7 and serves to provide the series electrical connection between the voltages generated through the peripheral portion of the bi-laminar structure and the center circular portion of the assembly. The mating surfaces of the discs 2A and 28 each having mating segmented electrodes 4 and 4A corresponding to electrodes 3 and 3A illustrated in FIG. 7. The polarization of the individual discs is indicated by the and signs in FIG. 7.

In order to complete the hydrophone assembly, refer again to FIG. 1. The tubular member 9 and tubular member 10 are machined to fit accurately within the machined peripheral surfaces of part 1 as illustrated. End caps with undercut peripheral webs, to permit fiexural suspension of the center piston portions 11 and 12, are provided to seal each end of the cylindrical housing 9 and 10. At each joint where the various mechanical parts are attached, an epoxy cement is preferably employed to make a permanent rigid waterproof bond. Mechanical connection is established from the pistons 11 and 12 to the common transducer element 2A and 2B by means of the connecting members illustrated in FIG. 1. In order to secure high rigidity for the connecting members, thinwalled tubular members 13 and 14 are used for the basic structure. At one end of tube 13 is attached an adapter 15 which is machined to seat flush against the under, flat side of piston 11 when the top of part 15 is assembled into the center hole in piston 11 as shown. Epoxy cement is preferably employed at the various mechanical joints throughout the assembly to achieve a rigid waterproof construction. At the opposite end of tubular member 13 an adaptor 16 is employed to provide coupling from member 13 to the center of the piezoelectric structure 2A and 2B. For deep water applications, the member 16 is preferably made of a high strength material such as alloy steel, in order that the total hydrostatic pressure acting on the diaphragms does not result in yielding of the material at the small diameter portion of the adaptor 16. A similar adaptor 16A connects tubular member 14 to the opposite side of the ceramic structure 2A and 2B as shown, and an adaptor 15A is employed for attaching tube 14 to piston 12.

A preferred method of assembly of the structure is as follows:

(1) Fabricate the sub-assembly comprising part 1 and the bi-laminar assembly 2A and 2B with the associated terminal leads and 7 as shown.

(2) Assemble parts 13, 15 and 16 as one composite structure using epoxy or any other suitable rigid cement.

(3) Assemble parts 9 and 11 and the sub-assembly 13, 15 and 16, employing a suitable fixture to ensure that the cylindrical tip of part 16 is held concentric with the periphery of part 9 during the curing of the cement.

(4) Attach sub-assembly comprising parts 1, 2A and 2B, to the open end of part 9. The cylindrical tip of part 16 will enter approximately midway into the center opening of the piezoelectric structure 2A and 2B. Apply epoxy or other suitable rigid cement within the center opening of part 2A and 2B and around the open peripheral edge of part 1. Attach part to part 1 and insert the tip of 16A of the sub-assembly 14, A and 16A into the center clearance hole through part 2A and 2B, permitting the ends of 16 and 16A to make contact through the epoxy cement that was placed in the opening. Allow cement to cure, using a fixture to maintain concentricity of the tip of part 15A with the periphery of part 10.

(5) Cement diaphragm 12 to seal the opening on part 10, allowing the pin tip of part 15A to pass through the clearance hole in part 12, as shown. Employ a shim 17, if necessary, to make up tolerance, using cement on both sides of 17, to provide a rigid bond.

After following the above steps, a complete waterproof hydrophone assembly will result which, when immersed in a sound field, will generate a voltage proportional to the differential pressure existing between diaphragm 11 and diaphragm 12.

If the hydrophone is placed in a sound field with the normal axis of the cylindrical structure facing the sound source, the sensitivity will be a function of frequency as indicated by the response curve in FIG. 8. The maximum sensitivity will occur at the frequency f The frequency f occurs when the length of the hydrophone is equal to one-half wavelength of the sound in the medium. At the lower frequencies, the sensitivity will decrease with decreasing frequency at the rate of 6 db per octave, as shown in FIG. 8. In order for the sensitivity characteristic to have the uniform slope shown in FIG. 8 over the frequency range of operation, it is necessary that the resonant frequency of the combined vibrating system, consisting of the transducer element 2A and 2B combined with the associated diaphragms and connecting rods, occur higher than the maximum frequency of desired use. Under this condition, the phase relationship between the generated voltage in the hydrophone and the sound pressure acting at the diaphragm will remain uniform among all the hydrophones that are manufactured, which will permit the use of the pressure gradient elements as direction finders in much the same manner as is achieved by the use of di-pole antennas.

For any sound arriving along a path which is at right angles to the axis of the hydrophone, the instantaneous sound pressure will be of the same magnitude and phase on the surface of each diaphragm and, therefore, zero voltage will be generated by the hydrophone. For sounds arriving at angles between 0 and the sensitivity will be proportional to the cosine of the angles as illustrated by the directional characteristic shown in FIG. 9.

Although I have chosen only a few specific examples to illustrate the basic principles of my invention, it will be obvious to those skilled in the art that numerous departures may be made from the details shown, and I, therefore, desire that my invention shall not be limited except insofar as it made necessary by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. In combination in a pressure gradient hydrophone transducer, a tubular housing structure having two open ends, a vibratile diaphragm attached to seal each open end of said tubular housing, a single transducer element capable of converting oscillatory mechanical pressures on said diaphragms into oscillatory electrical signals supported within and by said tubular housing at a location intermediate said two ends, mechanical connection means connecting each vibratile diaphragm to a common point, said common point being coupled to apply said mechanical pressures upon an unsupported region of said transducer element, and electrical terminal means connected to said transducer element.

2. In combination in a pressure gradient transducer, a tubular housing structure having two open ends, a multilaminar plate assembly, at least one element of said plate assembly comprising a transducer material capable of converting oscillatory mechanical stresses to alternating electrical signals, electrical terminal means connected to said transducer material, mechanical support means located within said tubular housing adapted for mounting a portion of said multilaminar structure, a section of the unsupported portion of said multilaminar structure being arranged near the center line of said tubular housing, a pair of vibratile diaphragms, each attached to seal an open end of said tubular housing, mechanical connection means attaching each diaphragm to a common point on the unsupported portion of said multilaminar structure whereby the diiference in alternating pressure impressed on the two vibratile diaphragms produce a resultant alternating force to vibrate the unsupported region of said multilaminar plate assembly causing an oscillatory electrical signal to be generated within said transducer material and to appear at the electrical terminals connected to said transducer material.

3. The invention set forth in claim 2 wherein said transducer is designed to operate in a sonic medium responsive to the receipt of sound waves having a frequency which is less than a predetermined maximum frequency, said transducer being characterized in that the resonant frequency of the transducer assembly is higher than said predetermined frequency of sound in the sonic medium for which L= 2 where L is the length of the tubular housing and k is the Wavelength at the predetermined frequency in the sonic medium.

4. The invention set forth in claim 2 characterized in that each of said mechanical connection means comprises a hollow tube with a solid plug attached to each end of said hollow tube and further characterized in that one end plug on each of said hollow tubes is rigidly bonded to opposite faces near the center portion of said multilaminar plate and the remaining end plug on each hollow tube is rigidly attached to the center of each vibratile diaphragm.

5. The invention set forth in claim 2, the stiffness of the vibrating system being controlled over the entire frequency range from the lowest audible frequencies to at least I kc., and the open circuit voltage sensitivity of the transducer increasing when used as a hydrophone for receiving sounds arriving along an axis parallel to the axis of the tubular housing, said open circuit voltage increasing with frequency at the rate of 6 db per octave over the entire low frequency range to at least 1 kc.

6. The invention set forth in claim 5 further characterized in that the directional pattern of the transducer in a plane containing the longitudinal axis of the tubular housing varies from a maximum for angles of incidence of the sound along the longitudinal axis to a minimum for sounds arriving at right angles to the normal axis of the transducer, the variation in sensitivity for other angles of incidence being approximately proportional to the cosine of the angle between the arrival path and the normal axis of the transducer.

7. In combination in a pressure gradient transducer, a hollow cylindrical housing having an opening at each end, a pair of piezoelectric discs, electrodes surfaces covering areas on both faces of said discs, bonding means for attaching said pair of discs together to form a bilaminar assembly, electrical terminal means attached to said electrode surfaces, mechanical mounting means within said cylindrical housing adapted for holding the periphery of said bi-laminar disc, bonding means for attaching said periphery of said bi-laminar assembly to said mechanical mounting means, a vibratile diaphragm attached to seal each of said open ends of said cylindrical housing, a pair of mechanical connection means one attached to each vibratile diaphragm near its center and the free ends of said pair of connection means coming together from opposite directions, bonding means for rigidly attaching said free ends from said mechanical connection means to a common region near the center of said bi-laminar assembly whereby the difference in alternating pressure impressed on the said pair of vibratile diaphragms is transferred to the center unsupported portion of said bi-laminar assembly causing alternating stresses to be set up in the piezoelectric plates which in turn generate alternating voltages across the electrode surfaces attached to said piezoelectric plates.

8. The invention set forth in claim 7 characterized in that said bi-laminar disc includes at least one plate of polarized ceramic material.

9. The invention set forth in claim 8 characterized in that the electrode surface comprises a circular conducting film placed to cover the center portion of the piezoelectric disc and further characterized in that the diameter of said electrode film shall not exceed about 55% of the unsupported diameter of the bi-laminar assembly.

10. The invention set forth in claim 8 characterized in that the electrode surface comprises an annular shaped film whose outer diameter is approximately equal to the unsupported diameter of the bi-laminar assembly, and further characterized in that the inner diameter of said electrode film shall not be less than approximately 55% of the unclamped diameter of said bi-laminar assembly.

11. The invention set forth in claim 8 characterized in that the electrode surface comprises two separated concentric areas, the separation occurring at approximately 55% of the diameter of the unsupported portion of the bi-laminar disc.

12. The invention set forth in claim 11 further characterized in that the region of the ceramic disc adjacent to the center electrode portion has the opposite polarity of the region of the disc adjacent to the outer annular electrode portion.

13. In combination in a pressure gradient transducer, a tubular housing structure, a bi-laminar piezoelectric disc assembly rigidly attached at its periphery to the inner peripheral region of said tubular housing, multiple separated electrode portions attached to the surfaces of said piezoelectric discs, electrical conductors connected to the exposed portions of said electrodes, the electrical polarizations of said piezoelectric discs being in such direction that the voltages generated through the separately electroded portions of said bi-laminar assembly when mechanical vibratory forces are imparted to the unsupported region of said bi-laminar element are in phase across the electrical conductors connected to the several electrode surfaces.

14. In combination in a pressure gradient transducer, a hollow tubular housing an opening at each end, a circular peripheral mounting means within the center portion of said tubular housing, a pair of polarized ceramic discs, said first disc having a separated electrode surface comprising a central circular portion and a separated annular portion on each of the two parallel plane faces of said disc, said second disc having a separated electrode surface on one of its faces identical to the separate configuration provided on the surfaces of said first disc and a continuous electrode area over its other opposite face, bonding means for rigid- 1y attaching said first and said second disc face to face with the identical separated electrodes making individual electrical connection through the mated face to face elec trode areas, two electrical conductor means, one connected to each of the concentric separated electrodes exposed on the unbonded face of said first ceramic disc, bonding means for attaching the periphery of said joined piezoelectric discs to the peripheral mounting means associated with said tubular housing, a vibratile diaphragm attached to seal each end opening of said tubular housing, mechanical connection means rigidly attached to each diaphragm, said mechanical connection means assembled rigidly together at their opposite ends and to the unsupported center section of said piezoelectric disc assembly whereby differential alternating pressures applied to the exposed diaphragm surfaces cause oscillatory mechanical forces to be imparted to the unsupported center region of said piezoelectric disc assembly.

15. The invention set forth in claim 14 and means for making the resonant frequency of the transducer assembly higher than the frequency of sound in the medium which corresponds to the frequency determined by the relationship L=)\/ 2 where L is the length of the tubular housing and A is the wavelength at the corresponding frequency in the medium.

16. The invention set forth in claim 14 characterized in that each of said mechanical connection means comprises a hollow tube with a solid plug attached to each end of said hollow tube and further characterized in that one end plug on each of said hollow tubes is rigidly bonded to opposite faces near the center portion of said multilaminar plate and the remaining end plug on each hollow tube is rigidly attached to the center of each vibratile diaphragm.

17. The invention set forth in claim 14 the stiffness of the vibrating system being controlled over the entire frequency range from the lowest audible frequencies to at least 1 kc., and means for increasing the open circuit voltage sensitivity of the transducer when used as a hydrophone for receiving sounds arriving along an axis parallel to the axis of the tubular housing, said open circuit voltage increasing with frequency at the rate of 6 db per octave over the entire low frequency range to at least 1 kc.

18. The invention set forth in claim 14 characterized in that the directional pattern of the transducer in a plane containing the longitudinal axis of the tubular housing varies from a maximum for angles of incidence of the sound along the longitudinal axis to a minimum for sounds arriving at right angle to the normal axis of the transducer, the variation in sensitivity for other angles of incidence being approximately proportional to the cosine of the angle between the arrival path and the normal axis of the transducer.

19. In combination in an electroacoustic transducer of the pressure gradient type for operation under water, a tubular housing structure having an opening at each end, a vibratile diaphragm attached to seal each opposite open end of said housing structure, rigid mechanical connection means contained within said tubular housing and rigidly attached to each of said opposing diaphragms whereby the hydrostatic pressure acting on the exposed surfaces of each of the two diaphragms results in opposing equal forces along the mechanical connection means thereby preventing any displacement of said vibratile diaphragms due to the influence of the hydrostatic pressure, a single transducer means supported within said tubular housing intermediate the ends thereof, said mechanical connection means rigidly attached to an unsupported region of said transducer means, and means responsive to differential alternating pressure acting on the diaphragms causing corresponding alternating forces to be applied to the unsupported portion of the transducer means for generating a corresponding alternating output voltage.

References Cited UNITED STATES PATENTS 2,647,162 6/1953 Duncan. 2,976,501 3/1961 Mattiat 340-8 3,054,084 9/1962 Parssinen et al 34010 X 3,056,104 9/1962 Kanski et al 340-8 X 3,187,300 6/1965 Brate 34010 2,414,756 1/1947 May.

RODNEY D. BENNETT, Primary Examiner.

CHESTER L. JUSTUS, Examiner. C. E. WANDS, B. L. RIBANDO, Assistant Examiners. 

1. IN COMBINATION IN A PRESSURE GRADIENT HYDROPHONE TRANSDUCER, A TUBULAR HOUSING STRUCTURE HAVING TWO OPEN ENDS, A VIBRATILE DIAPHRAGM ATTACHED TO SEAL EACH OPEN END OF SAID TUBULAR HOUSING, A SINGLE TRANSDUCER ELEMENT CAPABLE OF CONVERTING OSCILLATORY MECHANICAL PRESSURES ON SAID DIAPHRAGMS INTO OSCILLATORY ELECTRICAL SIGNALS SUPPORTED WITHIN AND BY SAID TUBULAR HOUSING AT A LOCATION INTERMEDIATE SAID TWO ENDS, MECHANICAL CONNECTION MEANS CONNECTING EACH VIBRATILE DIAPHRAGM TO A COMMON POINT, SAID COMMON POINT BEING COUPLED TO APPLY SAID MECHANICAL PRESSURES UPON AN UNSUPPORTED REGION OF SAID TRANSDUCER ELEMENT, AND ELECTRICAL TERMINAL MEANS CONNECTED TO SAID TRANSDUCER ELEMENT. 